Deep-drawn slidable foil based on polyolefins and polyamide

ABSTRACT

A thermoformable, puncture-resistant multi-layered film with surface slip, prepared by coextrusion and built up from polyolefin-based, polyamide-based and adhesion promoting polymer layers and its use for packaging sharp-edged goods, in particular foodstuffs.

BACKGROUND OF THE INVENTION

The surface slip of thermoformable films is becoming increasinglyimportant. For economic reasons, automatic insert-placing machines(robots) are used to insert products packaged in film into cartons. Anessential prerequisite for insertion by machines is a film with goodsurface slip. In the case of thermoformable films, the surface slip inthe thermoformed regions is critical. If the surface slip of the shapedparts is inadequate, then the carton cannot be filled with the intendednumber of packages. In addition, product protection by the packaging isan important factor. This means that the film has to have highmechanical load-bearing capacity, in particular towards sharp-edgedpackaged goods.

To simplify the description of the prior art and the invention, thepolymers which are used for films in the area of the present inventionand the abbreviations used for these polymers is explained in moredetail in the following, in order to simplify the rest of thedescription. If specific polymers are mentioned here, then these aresuitable for the corresponding layers in the film according to theinvention.

Polyolefins are referred to as “PO”. Examples of polyolefins are lowdensity polyethylene (LDPE), high density polyethylene (HDPE), linearlow density polyethylene (LLDPE), polypropylene (PP), polyisobutylene,polybutylene and all other polymers which are prepared from olefins.Furthermore, PO here also includes olefin copolymers consisting ofolefins and other monomers, wherein the olefins predominate in thecomposition. These are, for example, ethylene copolymers withunsaturated esters (e.g. vinyl acetate) and ethylene copolymers withunsaturated carboxylic acids or their salts.

Polyamides (abbreviated to “PA”) are understood in the widest sense tobe polymeric compounds which are linked together by the acid amide groupR—NH—CO—R′. Two groups of polyamides are differentiated: those built upfrom one parent substance by polycondensation of co-aminocarboxylicacids or polymerisation of their lactams to give polyamide-6 typecompounds and those which are produced from two parent substances,diamines and dicarboxylic acids by polycondensation to give polyamide-66type compounds. In addition, copolyamides are also known, these being acombination of the types of polyamide mentioned.

“PP” is used for polypropylene, no matter what the spatial arrangementof the methyl side-groups. “EPC” is used for copolymers of propylenewith 1-10 mol. % of ethylene, wherein the ethylene is randomlydistributed in the molecule.

“LDPE” is used for low density polyethylene which has a density in therange 0.86 to 0.93 g/cm³. LDPE molecules are characterised by a highdegree of branching.

“LLDPEs” are linear low density polyethylenes which contain, apart fromethylene, one or more α-olefins with more than 3 carbon atoms ascomonomers. The following are mentioned here as representative ofα-olefins: butene-1, hexene-1,4-methylpentene-1 and octene-1. Bypolymerisation of the substances mentioned, the typical molecularstructure of LLDPEs is produced, which is characterised by a linear mainchain with side chains attached thereto. The density varies between 0.86and 0.935 g/cm³. The melt flow index MFI is normally between 0.3 and 8g/10 min. In some publications, linear ethylene/α-olefin copolymers aresubdivided into VLDPEs and ULDPEs, in accordance with the density.Since, however, according to GNAUCK/FRÜNDT (Einsteig in dieKunststoffechemie Hanser-Verlag 1991, p. 58), the properties, processingand use of these copolymers corresponds largely to those of ethylenehomopolymers, the more precise differentiation is not used here.

“HDPE” is used to describe linear polyethylenes of high density whichhave only a small degree of branching in the molecular chain. Thedensity of HDPE may be between 0.9 g/cm³ and 0.97 g(cm³.

“mPE” is used here to describe an ethylene copolymer which waspolymerised using metallocene catalysts. An α-olefin with four or morecarbon atoms is preferably used as a comonomer. The polymers preparedusing conventional Ziegler-Natta catalysts frequently contain higherconcentrations of the α-olefins in the lower molecular weight fractions.As a result of the very uniform catalytic effect of metallocene centres,narrow distributions of molecular weights are found and, duringfractionation, very uniform incorporation of the α-olefins in both thehigh and also the low molecular weight fractions. The density ispreferably between 0.900 and 0.930 g/cm³. The molecular weightdistribution M_(w)/M_(n) is less than 3.5, preferably less than 3.

“EAA” is used for copolymers consisting of ethylene and acrylic acid and“EMAA” for copolymers consisting of ethylene and methacrylic acid. Theethylene content is preferably between 60 and 99 mol. %.

“I” is used to describe olefin-based copolymers in which the moleculesare cross-linked via ionic bonding. Ionic cross-linking is reversible,which results in breaking up of the ionic cross-linking at conventionalprocessing temperatures (180° C.-290° C.) and fresh formation in thecooling phase. Copolymers of ethylene and acrylic acid, which arecross-linked with sodium or zinc ions, are normally used as polymers.

“EVA” is used for a copolymer consisting of ethylene and vinyl acetate.The ethylene content is preferably between 60 and 99 mol. %.

“HV” is used for coextrudable, adhesion-promoting polymers (bondingagents). In contrast, adhesives are understood to be non-metallicmaterials, including polymers, which can bond two layers of film due tosurface adhesion and internal strength without substantially alteringthe structure of the bonded film layers. In contrast to bonding agents,adhesives are not coextrudable, but have to be applied separately bysurface application (e.g. lamination) or liquid application. Bondingagents are preferably modified polyolefins such as e.g. LDPE, LLDPE,mPE, EVA, EAA, EMAA, PP, EPC which are grafted with at least one monomerfrom the group of α,β-monounsaturated dicarboxylic acids such as, forexample, maleic acid, fumaric acid, itaconic acid or their acidanhydrides, acid esters, acid amides or acid imides. Other extrudablebonding agents which may be used are also copolymers of ethylene withα,β-monounsaturated dicarboxylic acids such as acrylic acid, methacrylicacid and/or their metal salts with zinc or sodium and/or theirC₁-C₄-alkyl esters, which may also be grafted with at least one monomerfrom the group of α,β-monounsaturated dicarboxylic acids such as, forexample, maleic acid, fumaric acid, itaconic acid or their acidanhydrides, acid esters, acid amides or acid imides. In addition,polyolefins such as e.g. polyethylene, polypropylene, ethylene/propylenecopolymers or ethylene/α-olefin copolymers which are grafted withcopolymers of ethylene with α,β-monounsaturated dicarboxylic acids suchas acrylic acid, methacrylic acid and/or their metal salts with zinc orsodium and/or their C₁-C₄-alkyl esters, may be used. Particularlysuitable bonding agents are polyolefins, in particular ethylene/α-olefincopolymers with grafted α,β-monounsaturated dicarboxylic anhydrides, inparticular maleic anhydride. HVs may also contain an ethylene/vinylacetate copolymer, preferably with a vinyl acetate content of at least10wt. %.

Different layers in a film are separated in the following by “/”.Mixtures of polymers in a layer are signified by round brackets andlinked by a “+”. Thus the film structure “ . . . /LLDPE/(mPE+LDPE)”describes a multi-layered film, wherein the outer layer contains amixture of mnPE and LDPE. The inner layer consists of LLDPE. The otherlayers in the film are indicated by “ . . . ”.

The following documents are regarded as the relevant prior art for thepresent invention:

EP 0 236 099 describes a 7-layered, coextruded film with a fullysymmetric structure. It includes a core layer made of an ethylene/vinylalcohol copolymer, two outer layers which each contain polymericmaterial or a mixture of polymeric materials, two polyamide layersbonding to the core layer and two adhesion promoting inner layers whichare located between the outer layers and the polyamide layers. The outerlayers are preferably linear polyethylenes. Films of this type are knownto be very tough and abrasion-resistant. No information is given withthe regard to the puncture-resistance of these films. Furthermore, filmsof this type can be sealed at relatively low temperatures. However, thisrepresents a contradiction, because low sealing temperatures can onlyever be achieved by using soft sealing layers. In the case of the fillysymmetric film structure, this would mean an abrasion-sensitive outerlayer, which produces a high resistance to friction.

EP 0 613 774 describes a multi-layered, readily thermoformable film withhigh puncture-resistance. The film has a polyamide on the outer face anda polymer based on olefins on the sealing face. The disadvantage of thisfilm structure is the external polyamide layer because good surface slipof the shaped parts against each other does not result immediately afterthermoforming.

WO 94/19186 describes a symmetric film with the film structurepolyolefin/bonding agent/PA/EVOH/PA/bonding agent/polyolefin. Thepolyamide is preferably an aromatic PA, or is derived therefrom, whichdoes not contain any caprolactam. The possibility that the outer bondingagent and polyolefin layers can be pulled away from the film isstressed. Aromatic or partly aromatic polyamides can only bethermoformed in an unsatisfactory manner because the thermoformingtemperature has to be higher than the glass transition temperature. Veryhigh glass transition temperatures of 100° C. and higher are known foraromatic and partly aromatic polyamides; thermoforming temperatures ofabout 70 to 90° C. are desirable. The possibility of thermoforming thesefilms and the surface slip properties of the films immediately afterthermoforming are not mentioned at all.

EP 0 686 497 describes the dissimilar sealing of different sealinglayers. Particularly good seals are obtained by means of a thinionomeric sealing layer, which is achieved by the auxiliary sealinglayer according to the invention consisting of two components, whereinone of the two components is an ionomer, elastomer, plastomer orcarboxyl-modified polyethylene. The surface slip properties of the filmsin general, in particular immediately after thermoforming, and thepuncture-resistance of the films are not discussed. It is known of thepolymers, ionomers, plastomers, elastomers and carboxyl-modifiedpolyethylene contained in the sealing layer, that they drasticallyreduce the surface slip of the films and lead to blocking of the films.Experience has shown that even the addition of antiblocking agents andlubricants can improve the surface slip characteristics of the filmsonly very slightly. In particular, the surface slip of films in thethermoformed regions is completely inadequate.

EP 0 800 915 describes 7-layered, coextruded film structures. The layersequence polyamide/ethylene/vinyl alcohol copolymer/polyamide is commonto all the film structures. Due to the method of manufacture (annularpolymer melts cooled with water), the special properties of clarity andprocessability are ascribed to these films. In the preferred form, thefilm contains polyamide as a non-sealing outer layer. However, it isknown that the surface slip properties of a film are impaired by watercooling. This applies in particular to the outermost polyamide layer.The surface slip can also be improved only marginally by addingadditives.

SUMMARY OF THE INVENTION

Thus, there is the object of providing a film designed in such a waythat

it is tack-free and

thermoformable and

has a sufficiently high surface slip in the thermoformed areas and

has a high puncture-resistance.

This is achieved in accordance with the invention by a multi-layeredfilm prepared by coextrusion, as shown schematically in FIG. 1, with afilm structure comprising

a) a first outer layer 10 consisting of one or more polyolefins orolefin copolymers or a mixture of these polymers and conventionallubricants and antiblocking agents,

b) a second outer layer 18 consisting of one or more polyolefins orolefin copolymers or a mixture of these polymers and conventionallubricants and antiblocking agents,

c) an inner layer 13 containing a polyamide or a mixture of severalpolyamides,

d) an inner layer 12 containing an adhesion-promoting polymer or anadhesion-promoting polymer mixture, based on polyolefins or alsocopolymers derived from polyolefins,

e) an inner layer 16 containing an adhesion-promoting polymer or anadhesion-promoting polymer mixture based on polyolefins or copolymersderived from olefins,

characterised in that the melting point of the second outer layer 18 islower than that of the first outer layer 10 and that an additional layer17 which contains a polyolefin or olefin copolymer or a mixture of thesepolymers is present between the second outer layer 18 andadhesion-promoting layer 16.

DETAILED DESCRIPTION OF THE INVENTION

The film according to the invention can be extruded on all conventionalextrusion devices on which polyamide or polyolefins can be processed.

Outer layer 10 contains polyolefins or olefin copolymers or a mixture ofthese polymers. LDPE, LLDPE, mPE, I, HDPE, PP, EPC or mixtures of thesepolymers are preferably used. To improve the surface slip, lubricantsand antiblocking agents are added. The amount of lubricant in layer 10should be between 50 and 2000 ppm, depending on the thickness of thelayer. The concentration of antiblocking agent in layer 10 should bebetween 50 and 3000 ppm, preferably between 100 and 2000 ppm.

The second outer layer 18 contains polyolefins or olefin copolymers or amixture of these polymers. LDPE, LLDPE, mPE, HDPE, PP, EPC, EVA, EAA,EMAA, I or mixtures of these polymers are preferably used. To improvethe surface slip, lubricants and antiblocking agents are added. Theamount of lubricant in layer 18 should be between 50 and 2000 ppm,depending on the thickness of the layer. The concentration ofantiblocking agent in layer 18 should be between 50 and 3000 ppm,preferably between 100 and 2000 ppm.

The melting point of outer layer 18 is lower than the melting point ofthe first outer layer 10 and should preferably be at least 5° C. belowthe melting point of the first outer layer 10. This improves the passagethrough machines of the films on thermoforming machines because thesealing layer facing the goods being packaged softens before outer layer10. Outer layer 10 is in contact with the packaging machine and thesurface slip properties are adversely affected in the event of softeningtoo soon.

Inner layer 13 preferably contains PA6, PA11, PA12 and PA66 orcopolyamides based on these or mixtures of these polymers as apolyamide. Copolyamides are e.g. PA6/66, PA6/12, PA6/6I, PA6/IPDI;PA66/610, PA6/11/66. To improve the oxygen barrier effect or thethermoformability, these polyamides may be mixed with partly aromaticPAs such as e.g. PA6I, PAMXD6, PA6I/6T.

The inner layer consisting of PA may be modified with additives.Suitable additives are e.g. crystallisation agents, lubricants andprocessing aids.

Furthermore, the polyamides may be modified by sheet-like particles. Thesheet-like particles may consist of organic or inorganic substances. Thecharacteristic diameter of the particles is understood to be thelongitudinal length of the particle, which is usually less than 700 nm.The oxygen barrier effect or the tensile strength of the film, forexample, can be improved by adding these particles.

Inner layers 12 and 16 contain an extrudable bonding agent. Modifiedpolyolefins or olefin copolymers are preferably used as these. Thebonding agent increases the bond adhesion between two otherwisenon-adhering layers.

Additional inner layer 17 consists of polyolefins and olefin copolymersor mixtures of these polymers. The thermoformability andpuncture-resistance, with adequate sealability, are affected in apositive manner if the melting point of layer 17 differs from themelting point of outer layer 18 by more than 3° C. Inner layer 17 andouter layer 18 exhibit sufficient adhesion to each other; an additionaladhesion-promoting layer is not required.

In a special embodiment of the invention, a further inner layer isplaced between inner layer 13 and adhesion-promoting layer 16, this alsoconsisting of a polyamide. The polyamide in this inner layer differs incomposition from the one in layer 13 This may be needed, for example, inorder to provide a cost-beneficial opportunity to improve the barriercharacteristics of the film to gases, in particular oxygen, orradiation, in particular ultraviolet radiation. In this connection, itis known that specific types of PA can absorb specific wavelengths of UVlight and thus are better able to protect sensitive products, such ase.g. meat.

Furthermore, in special cases of application of the invention, anadditional layer consisting of EVOH is placed between inner layer 13 andthe further inner layer consisting of polyamide, when the oxygen barriereffect which can be achieved with PA is not sufficient.

Films in accordance with the invention are suitable in particular forthermoforming applications. Thermoforming is understood to be theshaping of a film or sheet which is pressed or drawn into a pre-shapedmould, after warming, by means of a compressed air differential or apiston. During the thermoforming process, the rectangular relationshipof the film thickness decreases in accordance.

Films according to the invention are also suitable for applicationsknown as lap-seals. Here outer face 10 is sealed with outer face 18. Inthis case, to produce a high seal strength, both outer faces should havea similar composition in order to achieve a high degree of hot-tack.

Films according to the invention are suitable for packing a wide varietyof goods, whether they are present as one or several solid or liquidphases or a combination of the phases mentioned. They are suitable forpackaging cold or hot products.

To improve the surface slip properties, conventional lubricants andantiblocking agents may be added to the film. Examples of knownantiblocking agents are solid organic or inorganic particles whichproject from the outer surface of the sealing layer and improve thesliding behaviour in this way such as, for example, silicon dioxide,calcium carbonate, magnesium silicate, aluminium silicate, calciumphosphate, talcum and the like. Of these, silicon dioxide is preferablyused. Known organic antiblocking agents are, for example, incompatiblepolymers such as polyester or polycarbonate. Active amounts are in therange 0.05 to 2 wt. %, preferably 0.1 to 0.8 wt. %. The average particlesize is between 1 and 10 μm, wherein here particles with a sphericalshape are particularly suitable. These particles are preferably used inonly the two outer layers.

Other additives which improve the surface slip properties of the films,also when combined with the solid particles mentioned, are thesubstances usually called lubricants such as higher aliphatic acidamides, higher aliphatic acid esters, waxes, metal soaps andpolydimethylsiloxanes or other substances such as, for example, arementioned in Otto Pringer: Verpackungen für Lebensmittel, VCHVerlagsgesellschaft 1993, p. 53. The active amount of lubricant is inthe range 0.01 to 3 wt. %, preferably 0.02 to 1 wt. %. The addition ofhigher aliphatic acid amides in the range 0.01 to 0.25 wt. % isparticularly suitable. An aliphatic acid amide which is suitable inparticular for the outer layers of the film according to the inventionis erucic amide.

The total concentration of lubricant in the film should not exceed 1000ppm. With thicker films, lower absolute concentrations of lubricantshould be striven for, otherwise increased deposition of the lubricantat the surface of the film has to be expected. In the extreme case,optically detectable specks consisting of lubricant may appear on thefilm.

One or more layers in the film according to the invention may becoloured.

Films according to the invention may contain further additives such asantifog agents or antistatic agents. Known antistatic agents are,according to Hans Domininghaus: Kunststoffe und ihre Eigenschaften, p.23, VDI-Verlag, 1992, Düsseldorf, the so-called internal antistaticagents ethoxylated tertiary amines of fatty acids or ethoxylated glycolesters of fatty acids or else the external antistatic agents quaternaryammonium salts of fatty acids. Surface active substances which preventthe coating of the film surface with fine water droplets are calledantifog agents or also antimist agents. Known antifog agents are, forexample, glycerol esters of fatty acids or polyoxyethylenesorbitanemonooleate.

Outer layer 18 may be specified as a peel layer. A known process forachieving the peel effect is the addition of polybutylene topolyethylene or ethylenic copolymers such as are described in Ahlhaus:Verpackung mit Kunststoffen, Carl Hanser Verlag, 1997, p. 239

The total thickness of the film according to the invention shouldpreferably be a maximum of 400 μm, particularly preferably less than 300μm. The thickness of the polyamide layer, with respect to the totalthickness of the film, should preferably not exceed 40%. If the filmcontains EVOH, then the thickness of the EVOH layer, with respect to thetotal thickness of the film, should preferably be less than 10%,particularly preferably less than 7%. The sum of the thicknesses of thetwo bonding agent layers, with respect to the total thickness of thefilm, should preferably be less than 30%, particularly preferably lessthan 15%.

EXAMPLES

Preparing the Samples

All test samples were prepared on a multi-layer tubular blown film unit.The die temperature was about 220° C. and the widening ratio was about2.2. The take-up speed was between 9 and 17 m/min, depending on thethickness of film. The granules were mixed in accordance with the mixingratio and then metered to the extruders. The polymers are characterisedin accordance with table 1 below.

TABLE 1 Density Name [kg/l] Viscosity Comments PA-1 1.14 PA6, rel.viscosity of 4 (1% solution, measured in m-cresol) PA-2 1.19 PA6I/6T,about ⅔ isophthalic and ⅓ terephthalic acids PA-3 1.19 PA6/66, about 20%comonomers; rel. viscosity of 4.08 (1% solution, measured in 96% H₂SO₄)HV-1 0.915 2.5* LLDPE grafted with maleic anhydride HV-2 0.890 5** EPCgrafted with maleic anhydride LLDPE-1 0.920 1* LLDPE, comonomer: octeneLLDPE-2 0.940 1* LLDPE, comonomer: octene LLDPE-3 0.918 1* LLDPE,comonomer: butene LDPE-1 0.924 2* LDPE LDPE-2 0.924 0.8* LDPE LDPE-30.928 0.8* LDPE EVA-1 0.925 3* ethylene/vinyl acetate copolymer withabout 5% vinyl acetate EVA-2 0.928 2* ethylene/vinyl acetate copolymerwith about 4.5% vinyl acetate EPC-1 0.897 2** random propylene/ethylenecopolymer: melting point 142° C. EPC-2 0.900 8** randompropylene/ethylene copolymer: melting point 150° C. EPC-3 0.900 6**random propylene/ethylene copolymer: melting point 149° C. *measuredaccording to ISO 1133 at 190° C. and 2.16 kg **measured according to ISO1133 at 230° C. and 2.16 kg

The random propylene/ethylene copolymers all had ethylene contentsbetween 1 and 5 wt. %, wherein the temperature of the melting pointdecreases with increasing ethylene content. The density of the polymerswas determined in accordance with ISO 1183.

TABLE 2 Layer 10 Layer 12 Layer 13 Layer 16 Layer 17 Layer 18 Ex. 1EPC-2 HV-2 95% PA-1 + HV-1 LLDPE-1 66% LLDPE-2 + 5% PA-2 33% LLDPE-1 M.pt. 151° C. 149° C. 217° C. 121° C. 120° C. 124° C. Thickness 9 μm 4 μm20 μm 4 μm 10 μm 12 μm Comp. 3 95% PA-1 + HV-1 LLDPE-1 HV-1 PA-1 HV-167% LLDPE-2 + 5% PA-2 33% LDPE-1 M. pt. 217° C. 121° C. 120° C. 121° C.218° C. 121° C. 124° C. Thickness 10 μm 4 μm 10 μm 4 μm 10 μm 4 μm 18 μmEx. 2 EPC-2 HV-2 95% PA-1 + HV-1 LLDPE-1 67% LLDPE-2 + 5% PA-2 33%LLDPE-1 M. pt. 151° C. 149° C. 217° C. 121° C. 120° C. 124° C. Thickness20 μm 7 μm 40 μm 7 μm 13 μm 23 μm Comp. 4 95% PA-1 + HV-1 LLDPE-1 HV-1PA-1 HV-1 67% LLDPE-2 + 5% PA-2 33% LDPE-1 M. pt. 217° C. 121° C. 120°C. 121° C. 218° C. 121° C. 124° C. Thickness 25 μm 7 μm 19 μm 7 μm 105μm 7 μm 30 μm Ex. 5 LDPE-2 HV-1 PA-3 HV-1 LLDPE-3 EVA-1 M. pt. 110° C.121° C. 189° C. 121° C. 121° C. 104° C. Thickness 22 μm 7 μm 37 μm 7 μm18 μm 30 μm Comp. 6 EPC-3 HV-2 PA-3 HV-1 EVA-1 M. pt. 148° C. 149° C.189° C. 121° C. 104° C. Thickness 16 μm 7 μm 40 μm 9 μm 40 μm Ex. 7LDPE-2 HV-1 PA-1 HV-1 LDPE-1 EVA-2 M. pt. 110° C. 121° C. 218° C. 121°C. 121° C. 104° C. Thickness 45 μm 2 μm 75 μm 13 μm 41 μm 52 μm Comp. 850% LDPE- HV-1 PA-1 HV-1 PA-1 HV-1 EVA-2 3 + 50% LLDPE-3 M. pt. 118° C.121° C. 218° C. 121° C. 218° C. 121° C. 104° C. Thickness 43 μm 17 μm 35μm 16 μm 36 μm 17 μm 48 μm M. pt.: Melting point of layer Ex.: ExampleComp.: Comparison example

Methods of Measurement

Puncture-resistance was used to assess films which are subjected toattack by pointed items. Puncturing work is the work required in orderto pierce a stretched-out, membrane-like film. A circular sample with adiameter of 80 mm is taken from a film sample and inserted like amembrane into the sample-holder in an electronic tensile testingmachine. The sample-holder has a free diameter of 50 mm. The puncturingwork is determined from the integral of the force times the pathlengthwhich is required to pierce the sample. It is cited as N*cm. The testspike had a cone ground down to less than 90° with a diameter of 1 mm atits tip. The sample is pierced at a speed of 100 mm/min.

Thermoforming Tests

On a horizontal thermoforming machine (Tiromat 3000 from the Alfa LavalCo.) the films are subjected to a test approximating to the realsituation. The thermoforming behaviour of the films at a thermoformingtemperature of 90° C. is tested. The heating and shaping time is 3seconds. The size of the shaped part is 183 mm×114 nm (width×length),wherein the thermoforming depth is increased by 5 mm at a time until thefilm tears during the thermoforming process. The greatest depth at whichthe film does not tear is recorded.

Surface Slip of the Shaped Parts Against Each Other

On a horizontal thermoforming machine (Tiromat 3000 from the Alfa LavalCo.) the films are subjected to a test approximating to the realsituation. The thermoforming behaviour of the films at a thermoformingtemperature of 90° C. is tested. The heating and shaping time is 3seconds. The size of the shaped part is 184 mm×114 mm (width×length),the thermoforming depth is 60 mm. The bases of the shaped parts arerubbed against each other, using slight hand-pressure, immediately afterbeing thermoformed, by at least three test persons. If the shaped partscan be displaced with respect to each other, the surface slip issufficiently good and is labelled with “+”. If the shaped parts cannotbe displaced with respect to each other, they are awarded a “−”.

Determining the Melting Point of Plastics Films with the Melt TableMicroscope

The test method is used to determine the melting point of one layer in aplastics film. The melting point is understood to be the temperature atwhich the crystal lattice breaks down under atmospheric pressure. Thesamples being measured (a microtome section from the film) is heated ona heating table and observed through a microscope using polarisedtransmitted light. If the film consists of a partly crystalline, doublerefracting plastic material, it loses this property at the time itmelts, which can be observed using polarised light with the sampleinserted between crossed polarisation filters. If the film layer has tooweak, or no, double refraction, then non-polarised light can be used andthe temperature range over which melting onto a microscope slide takesplace is observed. In most cases, the melting point can be cited with anaccuracy of+/−1° C.

Max. thermo- forming depth Surface slip of dips Puncturing work Sample[mm] against each other [N*cm] Ex. 1 80 + 1.7 Comp. 3 65 − 1.6 Ex. 285 + 2.9 Comp. 4 80 − 2.5 Ex. 5 95 + 2.8 Comp. 6 95 + 2.6 Ex. 7 85 + 2.4Comp. 8 85 + 2.0

The comparison of the examples and comparison examples is chosen so thatthe films being compared contain comparable types and have a comparabletotal thickness.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an embodiment of the multi-layered Mm wherein:

a) is a first outer layer consisting of one or more polyolefins orolefin copolymers or a mixture of these polymers and conventionallubricants and antiblocking agents,

b) 18 is a second outer layer consisting of one or more polyolefins orolefin copolymers or a mixture of these polymers and conventionallubricants and antiblocking agents,

c) 13 is an inner layer containing a polyamide or a mixture of severalpolyamides,

d) 12 is an inner layer containing an adhesion-promoting polymer or anadhesion-promoting polymer mixture, based on polyolefins or copolymersderived from polyolefin,

e) 16 is an inner layer containing an adhesion-promoting polymer or anadhesion-promoting polymer mixture based on polyolefins or copolymersderived from olefins, characterized in that the melting point of thesecond outer layer 18 is lower than that of the first outer layer 10 andthat an additional layer 17 which contains a polyolefin or olefincopolymer or a mixture of these polymers is present between the secondouter layer 18 and an adhesion-promoting layer 16.

What is claimed is:
 1. A multi-layered film prepared by coextrusion,with a structure comprising a) a first outer layer consisting of: (1)one or more polyolefins or olefin copolymers or a mixture of thesepolymers; (2) lubricants; and (3) antiblocking agents, b) a second outerlayer consisting of: (1) one or more polyolefins or olefin copolymers ora mixture of these polymers; (2) lubricants; and (3) antiblockingagents, wherein the melting point of the second outer layer is lowerthan that of the first outer layer, c) a first inner layer containing anadhesion-promoting polymer or an adhesion-promoting polymer mixturebased on polyolefins or olefin copolymers or a mixture of thesepolymers, which is situated between the first outer layer and the secondinner layer, d) a second inner layer containing a polyamide or a mixtureof several polyamides, which is situated between the first inner layerand the third inner layer, e) a third inner layer containing anadhesion-promoting polymer or an adhesion-promoting polymer mixturebased on polyolefins or olefin copolymers or a mixture of thesepolymers, which is situated between the second inner layer and thefourth inner layer, f) a fourth inner layer containing a polyolefin orolefin copolymer or a mixture of these polymers, which is situatedbetween the third inner layer and the second outer layer.
 2. The film ofclaim 1 wherein the melting point of the second outer layer is at least5° C. lower than the melting point of the first outer layer.
 3. The filmof claim 1 wherein the polymers in the fourth inner layer adhere to thesecond outer layer and its melting point differs from the melting pointof the second outer layer by more than 3° C.
 4. The film of claim 1wherein it is tack-free.
 5. The film of claim 1 wherein the first outerlayer contains a member selected from PP, EPC, mPE, LDPE, LLDPE, HDPB, Iand mixtures of these polymers.
 6. The film of claim 1 wherein thesecond outer layer contains a member selected from PP, EPC, mPE, LDPE,LLDPE, HDPE, EVA, EMAA, EAA, I and mixture of these polymers.
 7. Thefilm of claim 1 wherein the second inner layer contains a polyamideselected from PA6, PA11, PA12, PA66, PA6/66, PA6/12, PA6/6I, PA66/11/66,PA6/IDPI, PA6I/6T, PAMXD6, PA6I and mixtures of these polyamides.
 8. Thefilm of claim 1 wherein the first inner layer contains polyolefins orolefin copolymers grafted with unsaturated dicarboxylic acids.
 9. Thefilm of claim 1 wherein the third inner layer contains polyolefins orolefin copolymers grafted with unsaturated dicarboxylic acids.
 10. Thefilm of claim 1 wherein the first outer layer and the second outer layercontain, as lubricant, higher aliphatic acid alludes, higher aliphaticacid esters, waxes, metal soaps or polydimethylsiloxane and, asantiblocking agent, solid organic or inorganic spacers.
 11. The film ofclaim 1 wherein a further inner layer is arranged between the secondinner layer and the third inner layer, this reducing the permeability ofgases or radiation.
 12. The film of claim 11 wherein an additional innerlayer consisting of EVOH is arranged between the second inner layer andsaid further inner layer.
 13. A method of packaging sharp-edged goodswhich comprises of wrapping the sharp edge(s) of said goods with thefilm of claim
 1. 14. The method of claim 13 wherein the sharp-edgedgoods are foodstuffs.
 15. The multi-layered film of claim 1, wherein theamount of lubricants in the first and second outer layer areindependently of each other are present in the range of 50 to 2000 ppmand the amount of antiblocking agents in the first and second outerlayer are independently of each other are present in the range of 50 to3000 ppm.
 16. The multi-layered film of claim 1, wherein the amount ofantiblocking agents in the first and second outer layer areindependently of each other are present in the range of 100 to 2000 ppm.17. The multi-layered film of claim 1 wherein: (1) the melting point ofthe second outer layer is at least 5° C. lower than the melting point ofthe first outer layer; and (2) the fourth inner layer adhere to thesecond outer layer and its melting point differs from the melting pointof the second outer layer by more than 3° C.